Metamorphism, Snow
The physical change of snow grains within the snowpack due to differences in temperature and pressure.

From the instant snow hits the ground, it begins an endless process of metamorphism. No commonly-occurring substance in nature undergoes such dramatic and rapid changes because snow exists near its “triple point”, meaning that solid, liquid and vapor phases all exist at the same time. In other words, small, subtle changes in temperature, pressure, humidity and temperature gradient can have a dramatic effect on the type of snow crystal that forms. This makes snow one of the most complex and changeable substances on Earth. Here is a condensed list of the most common types:

New snow Powder, rime, graupel, etc. No two are alike On the snow surface Falls from the sky
Rounded snow Equilibrium snow Old Snow Fine-grained, chalky Old layers of snow Low temperature gradient conditions (less than 1 deg C per 10 cm)
Faceted Snow Sugar Snow Kinetic Snow Depth Hoar (when near the ground) Sparkly, large-grained Anywhere in the snowpack Large temperature gradient conditions within the snowpack (more than 1 deg C per 10 cm)
Surface Hoar Frost, Feathers Sparkly, large-grained On the snow surface or buried by more recent layers Winter equivalent of dew on the snow surface
Melt-Freeze Snow Corn snow Spring snow Wet snow Corn snow Spring snow Wet snow Snow surface or buried by more recent layers Repeated melting and freezing of the snowpack